PROJECT SUMMARY At least 37% of patients treated in the emergency department (ED) are discharged without a definitive diagnosis, thus leaving the encounter with diagnostic uncertainty. A recent national survey of medical trainees found that 99% of trainees had experienced challenges discharging patients with diagnostic uncertainty, and 51% wanted formal communication training regarding uncertainty. Transitions of care are high-risk periods for patient safety, and effective communication between providers and patients is essential to promote patient safety during care transitions. Approaches for educating trainees and establishing competency around effective communication during diagnostic uncertainty are needed to improve the quality of communication at ED discharge, the most common transition of care. Simulation based mastery learning (SBML) is a competency-based educational approach that allows learners to develop skills through deliberate practice, resulting in mastery with little variation in outcomes. SBML is most often employed to develop mastery in medical procedures and has been shown to improve patient care practices. SBML offers a novel approach to developing and testing communication competency among medical trainees. The specific aims of this work are: SA1) To define key principles of uncertainty communication competency, SA2) To develop a simulation-based mastery learning curriculum to teach effective patient communication regarding diagnostic uncertainty and SA3) To test the efficacy of a simulation-based mastery learning curriculum in establishing competency in communication of diagnostic uncertainty among resident physicians. This work will be guided by an expert panel composed of patients and experts in patient communication, education, health literacy, diagnostic uncertainty, and simulation. Results and tools for implementing the SBML curriculum will be disseminated in the Communicating About Diagnostic Uncertainty (CADU) toolkit, with tools targeted to residency program directors and other education leaders via national list-serves. This project has potential to significantly impact the training and competency of providers in navigating safe and effective discharge communication for over one third of patients discharged from the emergency department.